Chemical sensitization of photographic emulsions



UNITED STATE for photographic emulsions Patented Nov. 15 1949 CHEMICAL SENSITIZATION OF PHOTO- GRAPHIC EMULSIONS Burt H. Carroll, Rochester, N. Y., assigno'r' to East'- man Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Application July 23, 1948, Serial No. 40,430

9 Claims.

1 This invention relates to sions and more particularly to chemical tizers therefor.

photographic emulsensi- The digestion of washed photographic emulthe chemical sensitization, has been regarded as probable, but there is little, if any, concrete evidence that reducing agents are capable of acting as chemical sensitizers. It has been demonstrated that sulfites can, under certain conditions such as relatively high silver ion concentration, act as chemical sensitizers and that the product of the reaction of sodium sulfite with silver bromide is probably metallic silver. Also the combined use of reducing agents such as arsenites with sulfur compounds for hypersensitizing has been disclosed. However, there are few specific compounds which act as chemical sensitizers other than those reacting to form silver sulfide. Stannous salts and alkali metal stannites, well known for their reducing properties, are described in the literature only as causing fog when used in connection with photographic emulsions for purposes other than sensitizing.

I have found that under the proper conditions stannous compounds act as chemical sensitizers in a manner comparable to sulfur compounds and that this activity can be superimposed on that of sulfur and natural gelatin sensitizers as well as optical sensitizers, particularly the latter.

Therefore, one object of my invention is to provide stannous compounds effective as emulsion sensitizers. Another object is to provide conditions under which stannous compounds are effective emulsion sensitizers. A further object is to provide photographic emulsions containing the novel sensitizers. Other objects will become apparent from the following description of my invention.

The objects of my invention are accomplished in part by incorporating into photographic emulsions small quantities of stannous compounds such as stannous halides. The exact type of stannous compound is not especially critical but stannous salts soluble in water but not readily 55 at 62 decomposed are preferred, and

because of the aerial oxidation of alkali metal stannites, these are less preferred because of the difficulty'of obtaining reproduceable results. Therefore, acid solutions of stannous salts such as stannous chloride, stannous bromide, iodide, fluoride and sulfate are preferred.

The stannous compounds are used in silver halide emulsions in quantities of the order of from 1 10 to 4 l 10 mol per 'mol silver halide. When using stannous salts such as stannous halides, especially stannous chloride or stannous sulfate, I can use a range of concentration of from about 1 x 10- to 20 10 mol per mol silver halide, preferably about 1 10 to 9 l0- mol per mol silver halide with the faster bromoiodide emulsions of the type shown in the examples below. When using alkali metal stannite solu- ,tions made from the stannous salts, I prefer to use from about lxlO- mol per mol silver halide up to about 44 l0 mol per mol silver halide, the higher quantities sometimes being required when the conditions of operation cause excessive aerial oxidation of the stannite solutions. Generally speaking, the lower ranges of concentration of stannous salts are used with the faster emulsions and up to about 20x 10- mol per mol silver halide with slower fine-grained emulsions except when using stannite solutions. Other factors well known to those skilled in the art will be found to influence the optimum concentration of stannous salt to obtain the characteristics desired for a given photographic emulsion. The appearance of fog with a given emulsion and concentration of stannous salt is a good criterion to follow indicating that too much salt has been used.

The following examples are given as illustrative of means of carrying out my invention:

Example 1 An acid stannous chloride solution was prepared by dissolving stannous chloride in water and adding a trace of mineral acid until the solution had a pH of approximately 3. A fast negative type non-optically sensitized gelatino-bromoiodide emulsion as normally prepared ready for coating, was treated with the varying amounts ofthe acid stannous chloride solution shown in the table following (Samples Nos. 3-7) at 40 C. for about 10 minutes before coating and drying on a support. Two samples of the original emulsion were coated as controls, the first sample No. 1 being treated for about ten minutes at 40 C. and sample No. 2 being treated for about five minutes 0. before coating. The sensitometric characteristics of the various samples were determined in the usual manner by exposure in an Eastman Type IIB Sensitometer and development in a developing solution of the following con- The characteristics of the various emulsion samples processed as above are shown in the following table: I

M01 SnOl X mli Sample N o. 10 per mol 8 eed Gamma Fog silver halide p 415 l. 37 0 435 l. 54 05 l. l 465 l. 41 04 2. 2 515 1. 41 05 4. 4 500 1.44 .05 6. 6 605 l. 48 O5 8. 8 575 1. 40 05 The other stannous salts of my invention can be used in a, similar fmanner to sensitize photographic emulsions.

Ezcample 2 An alkaline stannite solution was prepared by dissolving stannous chloride in water and adding suflicient sodium hydroxide to dissolve the oxide first formed. To samples of a moderately fast process type of silver halide emulsion as normally prepared ready for coating, were added varying amounts of the stannite solution shown in the table following. The samples and the original emulsion were coated and dried on a support, then tested as described in Example 1 above. The results of the test are as follows:

Mol sodium stannite Sample No. X 10- per l0/i Gamma Fog mol silver Speed halide during the preparation of an emulsion, either during a digestion step or after a preliminary digestion and washing before final melting and digestion of the emulsion, as desired. If preferred, the emulsion may be prepared by elimination of the washing step to produce the so-called unwashed type of emulsion.

The usual addenda such as optical sensitizers, antifoggants, spreading agents and hardeners may be used in the emulsions which are sensitized with the compounds of my invention and I am not limited as to choice of the vehicle for the emulsion in the respect that in addition to gelatin,

. other materials permeable to water such as polyvinyl alcohol, partially hydrolized cellulose esters such as partially hydrolized cellulose acetate and the like, may be used.

My invention having been described, I would have it understood that the preceding description is by way of example only and that my invention is to. be taken as limited :only by the scope of the appended claims.

What I claim is:

1. A light-sensitive photographic. emulsion comprising silver halide and from 1 l0 to 44 10 mol of a water soluble .stannous compoundv per mol of silver halide as a chemical sensitizing agent.

2. A light-sensitive photographic. emulsion comprising silver halidev and from l 1.0- to 20x10" mol of a water soluble stannous compound per mol of silver halide as a chemical sensitizing agent.

3. A light-sensitive photographic. emulsion comprising silver halide. and from 1 x 10" to 44 10 mol of stannite compound per mol of silver halide as a chemical sensitizing agent.

4. A light-sensitive photographic emulsion comprising silver halide and from'lx 10* to 20X 10- mol of stannous, salt per mol of silver halide as a, chemical sensitizing agent.

5. A light-sensitive photographic emulsion comprising silver bromide, and from 1 X 10- to 20 10 mol of Stan-nous salt .per mol of silver bromide as a chemical sensitizing agent.

6. Al-ight-sensitive photographicemulsion comprising silver bromide and from l X 10* to 9 10 mol of stannous salt per; mol of silver halide as a chemical sensitizing agent.

'7. A light-sensitivephotographic emulsion comprising silver halide and from 1 10 to 20x10- molof stannous halide per mol of silver halide as a chemical sensitizing agent.

8. A light-sensitive photographic emulsion comprising silver halide and from 1 l0- to 20x10- mol of stannous chloride .per mol of silver halide as a chemical sensitizing agent.

9. A light-sensitivephotographic emulsion com prising silver halide and from l X 10- 20 10- mol of stannous sulfate per mol' of silver halide as a chemical sensitizing; agent.

BURT H. CARROLL.

No references cited. 

